The Strait of Gibraltar, at its narrowest point, is about 14 km wide. Why didn't Germany blockade it during WW2?

It seems to me that only a handful of submarines and destroyers could do the job. Wasn't the German Navy (Kriegsmarine) capable of blockading it? Did submarine wolf packs at least hunt at that chokepoint?

The blockade would apply to US/UK/USSR ships, but allow Spanish and other neutral countries of course. Presumably this would not be a political problem for Spain.

Note: I'm not talking about the Gibraltar Fortress under British control. That's a different matter.

The UK would have loved for Germany to try to plant a bunch of their ships in one known place for an extended period of time. They spent most of that war scouring the oceans for them.
– T.E.D.♦Jun 14 '16 at 14:47

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But the problem is that the straights were so narrow. You're looking at it backwards. It wasn't that "they're so narrow they should have been able to blockade them", it was, "they're so narrow they are easily defended by surface vessels with sonar". There was (almost) no place to actually hide from those vessels and airplanes, and the straights are (relatively) shallow, so no u-boats traversed the straights during daylight hours unless they had no other choice.
– CGCampbellJun 14 '16 at 18:26

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An aerial picture of a submarine at periscope depth (strangemilitary.com/content/item/113453.html). Add to that that, during the night, radar and Leigh lights made it not too difficult to spot and attack submarines; in fact the germans had to develop the snorkels to be able to replenish oxygen without surfacing...
– SJuan76Jun 14 '16 at 18:29

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With the fortress Gibraltar intact to launch airstrikes against the subs? Are you mad? Seriously you must dispose of the fortress first, to which the only hope is the Italian battleships. If they can get within gun range of it (a whole 'nother can of worms) they can level it but otherwise ...
– JoshuaJun 15 '16 at 3:35

7 Answers
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...but if the Strait is guarded such that they can't get them without braving an enfilading strafe of torpedoes, then it wouldn't do much good even if they knew where they are.

From this comment by the OP, and others like it, it seems they don't appreciate the tactical limitations and vulnerabilities of a WWII submarine. I'll address that. While WWII submarines can be devastating to unescorted (or, earlier in the war, even escorted) merchant ships, they are extremely vulnerable to even small warships.

First thing to realize is WWII submarines aren't really submarines like modern submarines are. They are first and foremost surface ships. They spend most of their time on the surface where they can see (relying on lookouts with binoculars), move quickly, and use relatively cheap and plentiful deck gun ammunition to sink vulnerable merchant ships. It wasn't until the revolutionary Type XXI, completed at the war's very end, that the submarine would be able to spend the majority of its time under water.

Underwater a WWII submarine is very slow, has very limited battery power (a few hours at any kind of speed), limited breathing time, and must fire relatively short ranged (1000-5000 meters effective range), slow (20-40 knots), unguided, expensive torpedoes which are in limited supply and slow to reload. A Type VII submarine had only 14 torpedoes and just 6 tubes (4 bow, 2 stern). Once they were fired it could take an hour to reload. Hitting a slow, unaware, leisurely zig-zagging merchant ship was tricky enough and it usually took all 4 bow torpedoes to guarantee a hit. Hitting a fast, wildly maneuvering warship aware of your presence is almost impossible.

A German Type VII submarine, their most plentiful, could do 18 knots on the surface, but just 8 underwater. Surfaced, they had a range of 8500 miles, underwater just 80 miles... at a leisurely 4 knots. While submerged a target had to almost run them over to allow a torpedo attack, they could not chase them down.

While they carried an 88mm deck gun with 220 rounds of ammunition, this is a peashooter compared to what even the smallest destroyer carries. With a single gun, an unstable platform from which to shoot it (ie. the submarine rolls a lot), inferior range-finding gear, slow speed, and no armor... to be caught on the surface by even a well-armed merchantman was suicide. This was exploited by Q-Ships, armed merchantmen made to look like juicy deck gun targets.

If they stay deep they're safer, but also blind relying only on hydrophones (underwater microphones) to detect ships. If they want to use their periscope they must come up to periscope depth, shallow enough so their periscope can reach the surface. It's a tricky process to hold a WWII submarine at periscope depth. Come up a few feet and your scope is sticking up high out of the water and easy to be spotted. Come down a few feet and you can't see anything. The view isn't very good, being at the surface of the ocean and through a series of lenses.

Which brings us to the most underappreciated part of anti-submarine warfare (ASW): airplanes. We like to think of objects underwater as being effectively invisible, but from a high vantage point, like an airplane, a 200 foot long submarine can be seen quite easily at periscope depth. If an airplane spots a submarine on the surface it can be attacked with guns and bombs. At periscope depth it can be depth charged. To avoid attack, the submarine must dive deep to get away. Even if the airplane doesn't get the submarine, the submarine is now slow, blind, and losing battery power. With it's long loiter time, an ASW aircraft could hold a submarine down until a warship arrives to hunt it down with sonar.

With all that in mind, what happens when the German navy attempts to blockade the Strait of Gibraltar with submarines augmented with e-boats, destroyers and other light surface ships. To give it the best chance of succeeding, let's set it in the late summer of 1940 at the height of the Battle of Britain. The RAF is overstretched, the US hasn't entered the war, and ASW aircraft don't have radar capable of detecting submarine periscopes.

The German navy at this point is in bad shape. Never anything like a match for the Royal Navy, the war had started a few years too soon and naval production schedules were thrown into chaos. They had just taken a mauling in the otherwise wildly successful invasion of Norway losing many small ships.

At this point Germany only had 9 destroyers. They started the war with only 21, had since lost 12, and had only since commissioned one. They wouldn't last long with the major Royal Navy base of Gibraltar right there. It's unlikely Germany would risk its remaining overworked 9 destroyers on such a risky mission as blockading the Strait.

The Germans had E-boats, small, fast, expendable torpedo boats, and in some quantity. And, like a submarine, they were also extremely vulnerable to any real warship. Their short range precludes them from being used in this operation. Spain is staunchly neutral, and the south of France and Morocco are Vichy French territory who can still deny the Germans from using their ports.

While smaller ships have a hope of dodging the Royal Navy, larger surface vessels would be target practice. No German capital ship would risk staying in one area long enough for the RN to find them.

So no support from the German surface navy.

What about submarines? At this point in the war, the Germans had only commissioned 25 Type VII submarines of which only 13 remained. Their larger, ocean-going Type IX was too precious to be used in such an operation. At this point they had commissioned 11 but only 4 were left. It wasn't until 1941 that production would ramp up into the hundreds we think about when we think about the U-Boat war, but by then they were losing their technological edge.

Already, even in the summer of 1940, U-Boats were taking very heavy loses. And this was while mostly avoiding the Royal Navy. You're proposing they take them head on. Let's see what happens.

A submarine operating in the Strait of Gibraltar has all sorts of problems. It's a death trap. A submarine's single advantage and defense is stealth. This can be in not being detectable, but it's also not knowing where it's going to show up next. The enemy has to spread their ASW resources very thin looking for you. If German submarines try to blockade the Strait you know where you look and can concentrate your search in a small and restricted area. Only about 10 miles separates Africa from Europe. If detected there's nowhere to go. Being right next to a heavily fortified and extremely valuable British naval base means retaliation will be immediate and heavy.

A submarine that doesn't want to be detected in the Strait has a fighting chance... if they're moving into the Mediterranean west to east with the currents. The mixing salinity between the Atlantic and Mediterranean made underwater detection difficult, and the currents could be used to transit the Strait silently. It was still a very, very dangerous transit. Out of 62 U-Boats which made the trip from the Atlantic to the Mediterranean, 9 were sunk and 10 turned back. None returned.

But our submarines are on the attack, and a submarine running deep cannot see or attack. Our submarines have to spend their nights on the surface charging batteries and hunting, and their days either submerged avoiding aircraft and saving battery power, or hunting at periscope depth, but also vulnerable to ASW aircraft. Stuck in a narrow box of ocean, they're easy pickings for ASW aircraft and ships who can patrol over the area again and again and again until they've hunted them all down.

In addition to ASW hunters, any merchant ships transiting the Strait can be formed into convoys and protected by heavy ASW escort. Since the submarines are bunched up in the Strait, the same ASW fleet can protect all convoys moving through the gap. Submarines would not be allowed unescorted targets.

Attacking an escorted convoy would be suicide. Even with surprise, even with a wolf pack, avenging ASW aircraft and ships are never far away at Gibraltar. Every attack would pinpoint the submarines' position, and at 8 knots submerged they can't get far before an ASW vessel or aircraft shows up to search.

As for larger Royal Navy ships which might be worth the risk, they will either avoid the Strait until it is cleared, or be heavily escorted and moving at high speed. Capital ships have little place in an ASW fight except to be targets.

To sum up why they didn't try this...

Submarines are extremely vulnerable to even the smallest warship.

Submarines have to be surfaced most of the time.

Submarines must be surfaced or near the surface to attack, vulnerable to ASW aircraft.

The Germans would only be able to muster a dozen submarines.

No support from the surface navy.

ASW search would be relatively easy in the restricted area.

Convoys could be heavily protected in the restricted area.

ASW ships and aircraft were close by at Gibraltar.

The British would react very violently.

An attempt by the Germans to blockade the Strait would be a dream come true for the British. It would draw the entire German submarine fleet away from their very hard pressed Atlantic merchant convoys, and into a fixed area where they can be hunted and destroyed by a relatively small number of cheap, short-ranged ASW warships and aircraft.

For an appreciation of the troubles of a WWII submarine, I recommend the famous fictional but quite accurate German submarine movie Das Boot. They even have to transit the Strait.

I would also highly recommend the historical book One Of Our Submarines, the account of British WWII submarine captain Edward Young. This is an accurate, detailed, and very well written biographical account of submarine life and tactics. He also has to deal with straits, and without the luxury of deep water.

Thank you, you pretty much attacked the answer from every possible angle. Sadly I've already seen U-571 but I assure you I do not trust Hollywood and my question did not spawn from it. My question spawned from curiosity as to why such a narrow waterway was not blockaded. (This is about to lead me to ask why the British did not blockade it.) I watched a fraction of Das Boot but could not finish it due to culture clash, maybe I'll try again. I'll add the book to my ever-growing wishlist stack and maybe get to it a few years from now.
– DrZ214Jun 14 '16 at 20:27

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@DrZ214 Gibraltar was effectively blockaded by the British, but except for submarines it didn't affect the Axis much. For supplies to North Africa the Axis could use much shorter route from Italy and Greece. The Italian navy was already in the Mediterranean and wasn't designed to leave it. The German surface navy was either bottled up in port or out commerce raiding.
– SchwernJun 14 '16 at 20:45

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This was an absolutely amazing answer, once of the best I've seen on stack* in a long time.
– DaveJun 14 '16 at 23:22

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A very good answer, dispelling the misconception many people have about WW2 submarines being similar to those seen in "the Hunt for Red October". Comparing a WW2 submarine to a late cold war submarine is like comparing a rickety wooden WW1 fighter aircraft with a modern jet fighter. Basically, WW1 and WW2 submarines were just torpedo boats with the additional capability to briefly submerge. It would have been nice if you had pointed out that WW2 torpedoes were unguided, therefore only effective in a surprise attack. A topic I missed was what if the Germans also used aircraft to counter ASW.
– vszJun 15 '16 at 6:11

Gibraltar during the war had a quite formidable British naval presence (Force H), an airfield, and significant coastal gun emplacements easily capable of covering the entire strait.

The primary batteries were a set of twin 9.2" naval guns guns at the southern end of the peninsula, which had sufficient range to interdict all surface naval traffic through the straits. Six more single-gun batteries of 9.2" naval guns were placed facing seaward to the east. Finally, a twin 9.2" howitzer battery faced the straits as well.

The secondary batteries were just as numerous, with eight 6" naval guns, although three of these faced northward, toward the border with Spain. A tertiary armament of four 4" guns in two batteries protected the eastward cliffs.

Trying to blockade the strait would not have gone unopposed, and would likely be impossible without attacking / suppressing / capturing the fortress itself.

For which Germany had plans (Operation Felix), which however were postponed "until after the defeat of Soviet Russia".

At all times during the war, the German navy -- being significantly outnumbered -- tried to avoid engagements with the Royal Navy, not go looking for them. German warships and submarines were tasked to attack convoys, disrupt overseas trade, or protect land-based operations (Norway). At no point was a head-to-head confrontation with the Royal Navy a winable prospect for Germany.

Attacking the Strait of Gibraltar, then staying there and waiting for British reinforcements, would have been several flavors of foolish.

From the comments, the OP asked if it wouldn't be possible for "one or two dozen submarines" to blockade the strait.

Submarines in WWII are hit-and-run weapons. With extremely limited range and speed while submerged, they are required to operate at the surface (or, in case of the later snorkel-equipped boats, near the surface). They close in surfaced, attack at night or from submerged ambush, and then avoid enemy escorts. Once detected, a submarine is firmly on the defensive.

"Blockading" submarines would have to operate close to the coast, the enemy naval base, and most importantly enemy airfields. They would lose the element of surprise, would be actively searched for (a submarine close to the surface is easily spotted from the air, a deep-diving submarine isn't blockading), and once spotted they would be hunted down.

Even against an outdated destroyer a submarine is at a stiff speed disadvantage and cannot escape. In the open seas it can hope that the destroyer needs to keep up with the convoy, or is unwilling to expend its last depth charges once the convoy has moved on. With no convoy to escort and its naval base nearby, a destroyer can take all the time in the world to hunt the sub, and can be relieved once it has spent its depth charges. The sub would be sunk.

And Germany did not have "one or two dozen" submarines to spare in the first place. They would have to be taken from other theaters of operations, completely opening up the North Atlantic for US-GB and US-USSR convois for example.

Blockading Gibraltar with submarines is a non-option tactically, and would not have made sense strategically, given available resources.

Another good example is the Battle of Crete. The Germans had air superiority while the British had naval superiority. It did not go well for the Navy.
– T.E.D.♦Jun 14 '16 at 14:51

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@T.E.D.: Crete didn't go well for the Luftwaffe or the Fallschirmjäger either... I wanted to list cases where the Kriegsmarine -- if not actually ordered to cover land-based operations -- preferred to avoid the RN instead of picking a fight.
– DevSolarJun 14 '16 at 14:53

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Given the disparity in strength between the Royal Navy and the German navy, the only way for German to survive for long in the Atlantic was to use hit-and-run tactics. That precludes "A few destroyers in the back for good measure..." U-boats themselves were meant for hit-and-run tactics. Once a U-boat was spotted by the British, it had to flee or die. Parking a bunch of them together in a known piece of sea near a large Naval/Air base would have been suicide.
– Gort the RobotJun 14 '16 at 16:52

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You're talking a war of attrition between the Royal Navy and the German U-boat fleet. This would have ended with a slightly smaller Royal Navy and no German U-boat fleet, which would have resulted in lots of happy convoys coming from the US. You seem to be under the impression that a U-boat was invincible when up against a destroyer. That is far, far from the case.
– Gort the RobotJun 14 '16 at 17:31

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@DrZ214 Don't overlook the fact that the Brits would also be using aircraft, not just ships, to reopen the straits. In WW II ASW was already a combined arms effort.
– KorvinStarmastJun 21 '16 at 21:58

While you are at it why not blockade the port of London?
Isn't that a better target?
You could come in with 20 subs get 40 kills and go out in a blaze of glory as all the subs are spotted and sunk.

The real reason is air power and counter attacks.
Submarines are expensive. If you kill one merchant ship and lose the sub that killed it, it's a net loss for you. The sub needs to kill and live to kill again. WWII subs could only stay underwater for a few hours, could not move very fast, and they have weak armament compared to a surface ship.

Also, the German navy was too weak to win a confrontation with the Royal navy, so subs had to rely on hit and run.

Airplanes are a real problem because they can race to the area of an attack and have enough range they can find the sub and start bombing it.
The allies deliberately bent their supply lines north to keep the merchant ships under air cover from Greenland and Iceland as long as possible since subs normally attacked during gaps in air cover.

Blockading Port of London, even if successful, accomplishes nothing as there are 4+ other big ports at Britain. Blockading Strait of Gibraltar would deny the Mediterranean which is a very strategic area as evidenced by allied operations in Africa/Italy/Greece (except the Suez Canal, but to bring a fleet all the way around Africa, and its resupply fleet, would make it 20x slower).
– DrZ214Jun 14 '16 at 20:46

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@DrZ214 I believe the point was that both are suicide missions.
– SchwernJun 14 '16 at 20:47

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@DrZ214 The Panama Canal is a plausible submarine target. Instead of maintaining a blockade you need only damage the locks and dams with a small and quick strike. Both the Germans (Operation Pelikan) and Japanese (with specially built I-400 submarines) planned such a surprise strike with aircraft flying from submarines. Though, as you correctly point out, by the time the Axis had the ability to attack the Canal its value as a target diminished.
– SchwernJun 14 '16 at 20:51

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The point of my somewhat snarky opening line was that it was a suicide mission, not so much about the stratigec value of the targets. Schwern is right.
– sdrawkcabdearJun 14 '16 at 20:57

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@DrZ214 The Japanese, probably optimistic, estimate was a successful attack against the Gatun Locks by 10 aircraft each carrying a single 800 kg bomb would put the canal out of action for six months. By 1944 security at the canal had gotten lax since only a conventional surface strike was considered plausible, and surprise was to be achieved by the aircraft attacking from the Atlantic side. Here's a good documentary on the Japanese plan.
– SchwernJun 14 '16 at 21:10

I'm not a historian nor a scholar of sorts, but I was lucky to be at the right place at the right time and found out from one of the most capable people who could answer this question.

I lived in Madrid, Spain in the late 60s, early 70s. My father grew up as a teenage Nazi in WWII. Not his choice but actually my grandfather's. My grandfather wasn't a Nazi either. They were Hungarian and my grandfather had been a professor at the university of Budapest. As the war broke out the Nazis came to him (and I guess others) and offered to take his family to live in Germany if they so wished. My grandfather, a professor in what today would be political science, had a choice of going to Germany or wait for the Russian communists to come and control Hungary. He chose the former. My father out of social pressure grew up loving Hitler till his last days. The German culture was closer to his Austro-Hungarian.

While living in Madrid, he found out that Otto Skorzeny lived literally a stone's throw away from us. I was in high school in those days.

My father invited him over one day. He was old, his hands shook as he smoked. He held his cigarette between his forefinger and thumb with his palm facing up. In this position he brought it to his mouth.

He had written a book, a copy of which he gave to me but wouldn't sign. I was not a Nazi. Skorzeny made a remark then about Hitler not taking Gibraltar. He said Hit!er was not a military trained person. Skorzeny was. Along with him, other people pushed Hitler to control Gibraltar, but in vane. He remarked that a lot of German forces could have been freed to fight elsewhere rather than in Africa. Skorzeny said precisely as questioned above, that control of the entrance to the Mediterranean was a most important point to win the war. Hitler chose not to control the entrance to the Mediterranean. Skorzeny was the most feared man in Europe. He liberated Mussolini, among his other feats.

-1 personal anecdotes, very partially related to the subject, it is more like a comment than an answer. Please participate with answers, what you posted here could fit in a comment, please delete, and get the proper reputation for that.
– CsBalazsHungaryJun 15 '16 at 8:11

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Nice anecdote, but Skorzeny was talking about taking Gibraltar, not blocking the strait from the sea.
– DevSolarJun 15 '16 at 8:11

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@CsBalazsHungary: But it really is the answer. The med is the key to victory, and Gibraltar is the key to the med by one way or another.
– JoshuaJun 15 '16 at 16:13

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I am absolutely willing to revoke -1 and eventually turn it to +1, if you link or quote a supporting source. What I found about Skorzeny is generally about the war against Britain. Couldn't find Gibraltar reference.
– CsBalazsHungaryJun 16 '16 at 12:08

Blockade requires local superiority

To enforce a blockade at a chokepoint, you need to arrive there and stay there, winning any engagements. To win local engagements, you need to be stronger than an expected blockade-breaking force. WW2 Germany simply could not do that.

A blockade, no matter if it is done by a handful of subs, or a handful of destroyers, or the whole Kriegsmarine, would result in arrival of a superior British navy force and loss of the involved German ships without much benefit. The German naval weakness meant that their main option was commerce raiding by unpredictable surprise attacks that avoid confrontation with UK navy, and the British goal was to try and force a confrontation to eliminate their ships as German ability to replace them was very limited.

If there was a known location to encounter German ships such as a blockade of Gibraltar, then the British navy would be very happy to take on this offer.

The Germans lost 10 destroyers in the naval battles surrounding the conquest of Norway in 1940. These losses represented half of the modern destroyers constructed by Germany after WWI. It took Germany several years to construct replacement vessels.

A blockade of Gibraltar would have been difficult to maintain without the involvement of neutral Spain to the north and Spanish Morocco to the south.
With no air cover, any German vessels patrolling the Straits would have been sitting ducks for the British forces either in the Mediterranean or the eastern Atlantic Ocean.

Many folks forget Great Britain had a very formidable submarine force in World War 2 too. What made U-Boats such a menace was the Fall of France and the opening of the Ports of Brittany to the Kreigsmarine. This opened up the Atlantic but not the Med as there were and in fact still are three entrances to the Med... Gibraltar, Suez and the Bosporus. Germany had alliances with Spain, Italy and Turkey therefore Great Britain with whom she was at War with on her own posed no threat to the 3rd Reich after the Fall of France in 1940.